The present invention relates to a hydraulic control system for a four-wheel drive torque transfer mechanism having a two-wheel drive direct-coupling gear, four-wheel drive direct-coupling gear and reduced-speed four-wheel drive gear.
A typical torque transfer mechanism of this type is connected to the output shaft of the transmission of a power source and is adapted to distribute power to the front axle and the rear axle of a vehicle through the output shafts thereof. While a four-wheel drive vehicle equipped with such a transfer mechanism is running straight ahead, the output shafts of the transfer mechanism for distributing driving force to the front and the rear axles are connected with each other through power transmitting means, such as gears, chains or belts, at a predetermined ratio of revolving rate. When this vehicle is turned by controlling the steering wheels, usually, the front wheels (the front wheels are supposed to serve as steering wheels, hereinafter, in this specification), differential rotation between the respective output shafts of the transfer mechanism for driving the front axle and the rear axle is required, since the turning radius of the front wheels is greater than that of the rear wheels.
Accordingly, a conventional transfer mechanism of this type comprises a differential gear mechanism in the transfer gear unit, whereby either the output shaft for the front axle or the output shaft for the rear axle is directly connected to the output shaft of the transmission while the other output shaft of the transfer mechanism is connected to the aforementioned output shaft through a differential gear mechanism. A vehicle equipped with such a driving mechanism is liable to fail running when one of the wheels fails to keep the traction resulting from bad contact with the ground during four-wheel drive running, since differential gear mechanisms are mounted on the front axle and the rear axle, respectively. In order to avoid such running inability of the vehicle, a viscous control coupling is formed in the differential mechanism of the transfer gear unit between the two output shafts thereof or a so-called limited slip differential mechanism is employed so that at least some torque of a magnitude can be transmitted to the four wheels.
However, it is usual to provide in the transfer gear unit an additional mechanism capable of locking the driving train connected to the front and the rear axles to attain nonslip rotation, since there still remains the possibility of slipping in case of insufficient torque transfer capacity of the mechanism even when the viscous control coupling or the nonslip differential mechanism is employed. Although the safety and the durability of the transfer mechanism are increased and the riding comfortableness of the vehicle is improved through the provision of the differential mechanism, viscous control coupling or limited slip mechanism, and the locking mechanism, the cost of the vehicle increases.